1. Field of the Invention
The present invention relates to an optical scanning device that scans light beams to write and form an image for use in an image forming apparatus, such as a printer, a facsimile machine, and a copying machine, and more particularly, to an optical scanning device suitably used in a color image forming apparatus, such as a color laser printer and a digital color copying machine, and an image forming apparatus equipped with the same.
2. Description of the Related Art
An image forming apparatus in the related art using the electrophotographic method, such as a copying machine and a printer, is equipped with an optical scanning device that scans a light beam modulated according to input image data on the surface of a photoconductive drum charged uniformly by the charger. An image is formed by developing an electrostatic latent image formed by the optical scanning device into a toner image by the developing device, and then transferring the toner image onto a recording sheet of paper or the like.
Incidentally, as a color image forming apparatus is becoming faster, a digital copying machine or a laser printer adopting a method (so-called 4-drum tandem method) is now put into practical use, in which, for example, four photoconductive drums are aligned in the transportation direction of recording sheets of paper to form electrostatic latent images by exposing the photoconductive drums to light simultaneously using plural optical scanning devices corresponding to the respective photoconductive drums, and after these electrostatic latent images are developed into toner images by the developing devices that use developing agents in different colors, such as black, magenta, cyan, and yellow, these toner images are transferred onto the same recording sheet of paper successively so as to be superimposed one on another for a color image to be obtained.
According to the 4-drum tandem method, because an image can be outputted at the same speed in either color printing or monochromatic printing, it is advantageous when fast printing is desired. However, because four optical scanning devices are provided correspondingly to four photoconductive drums to expose the drums, the apparatus tends to increase in size. Meanwhile, to meet the need for a compact image forming apparatus in recent years, there has been proposed an optical scanning device that is made compact by configuring in such a manner that plural light beams emitted from the light sources provided separately for respective colors are deflected by a single deflector, so that the light beams are guided to different photoconductors to expose and scan the corresponding photoconductors (for example, see JP-A-2000-180750).
The optical scanning device of a type proposed in JP-A-2000-180750 is configured in such a manner that light beams go incident on the deflector at different angles in the sub-scanning direction for making it easier to separate optical paths of plural deflected light beams. This gives rise to bows (a phenomenon that scanning lines of light beams deflected by the defector are curved) of different quantities in the respective light beams deflected by the deflector. Because the quantities of the bows vary among the respective light beams, a color shift occurs in the resulting color image.
Such being the case, JP-A-2005-288825 proposes an optical scanning device that makes a color shift or an image curve in the color image caused by bows occurring in the light beams hardly noticeable by setting, of plural light beams corresponding to developing agents in different colors, a smaller angle of incidence on the deflector for light beams corresponding to developing agents having high visibility, that is, low brightness while setting a larger angle of incidence on the deflector for light beams corresponding to developing agents having low visibility, that is, high brightness.
For the optical scanning device of a type proposed in JP-A-2005-288825, it is, however, necessary to dispose plural folding mirrors as reflection members inside the optical scanning device to guide light beams to the corresponding photoconductors. The folding mirrors are disposed inside the optical scanning device with the both end portions being held so as not to interrupt the optical paths of the light beams. However, because the folding mirrors are held at the both end portions alone, they eventually bend due to their own weights and the reflection surfaces are curved. The scanning line of a light beam reflected on the bent folding mirror is curved as well.
In addition, for the reason of setting the optical paths of light beams, plural folding mirrors provided inside the optical scanning device are installed at different angles. The magnitude of the bend occurring in the folding mirror varies with the angle at which the folding mirror is installed. The magnitude becomes larger as the angle of installation of the reflection surface becomes closer to horizontal, and the magnitude becomes smaller as the angle of installation of the reflection surface becomes closer to vertical. Accordingly, curves of different magnitude occur in the scanning lines of the light beams reflected on the folding mirrors installed at different angles.
Generally, a correction lens furnished with an optical face tangle error correction capability is provided somewhere in the middle of each optical path of light beams, and the curve of the scanning line of a light beam as described above is corrected by allowing the light beam to pass through the correction lens. However, in a case where a curve of different magnitude occurs in the scanning line of each light beam after the light beam passed through the correction lens due to a difference of the angle of installation among the folding mirrors as described above, the curve thus occurred is not corrected and the curved scanning line directly scans the photoconductive drum. This poses a problem that a color shift resulting from displacement of the scanning line occurs in the formed color image.